The solid complex is washed with acetonitrile (50 mL). This acetonitrile solution is discarded because of the low ee (~80% ee of the S-enantiomer is contained). The resulting solid complex (96% ee, R-enantiomer) is transferred to a 250-mL flask. Methanol (100 mL) is added, and the resulting suspension is refluxed for 24 hr to upgrade the enantiomeric excess to >99% ee. After the mixture is cooled to room temperature, it is filtered and the solid washed with methanol (20 mL). The solid complex is suspended in a mixture of ethyl acetate (300 mL) and 1 N HCl (150 mL) and stirred until complete dissolution occurs (0.5 hr). The solution is transferred to a separatory funnel, and the organic layer is separated and then washed with 1 N HCl (150 mL) and brine (150 mL). The organic layer is dried over Na2SO4, filtered, and concentrated to an off-white crystalline solid [9.83-10.16 g, 85-88% recovery, mp 206-207°C, >99.8% ee of the R-enantiomer [α]21
D
26.2~30.9° (THF, c 1)] (Note 4), (Note 5), (Note 6).

2.
The enantiomeric excess of 1,1'-bi-2-naphthol in the filtrate at room temperature is 98.6% and at 0-5°C 99.0%.

3.
These acid washes are to remove residual N-benzylcinchonidinium chloride in the filtrate.

4.
Numerous chiral HPLC columns have been used for determination of chiral purity of 1,1'-bi-2-naphthol.2,3 The submitters used Diacel Chiralpak OP(+) column (4.6 mm × 250 mm) at room temperature for their chiral assay. Typical retention times of 1,1'-bi-2-naphthol are 14 min (R-enantiomer) and 20 min (S-enantiomer) using methanol as an eluting solvent at 0.5 mL/min. The submitters' detection limit of minor enantiomers is about 0.1%. The checkers used a Pirkle covalent D-phenylglycine column using isopropyl alcohol:hexane (5:95) as the eluting solvent at 1.0 mL/min with UV at 312 nm.

The procedures in this article are intended for use only by persons with prior training in experimental organic chemistry. All hazardous materials should be handled using the standard procedures for work with chemicals described in references such as "Prudent Practices in the Laboratory" (The National Academies Press, Washington, D.C., 2011 www.nap.edu). All chemical waste should be disposed of in accordance with local regulations. For general guidelines for the management of chemical waste, see Chapter 8 of Prudent Practices.

These procedures must be conducted at one's own risk. Organic Syntheses, Inc., its Editors, and its Board of Directors do not warrant or guarantee the safety of individuals using these procedures and hereby disclaim any liability for any injuries or damages claimed to have resulted from or related in any way to the procedures herein.

3. Discussion

Both enantiomers of 1,1'-bi-2-naphthol are widely used for various applications: 1) chiral inducing agents for catalytic, asymmetric reactions such as the Diels-Alder reaction,4 ene reaction,56 or as Lewis acids;78 2) enantioselective reduction of ketones;910 3) synthesis of chiral macrocycles1112 and other interesting compounds.13 Previously reported resolutions include: 1) making a cyclic phosphate of binaphthol, then resolution and subsequent reduction to release the pure binaphthol;14215161718 2) using enzymatic hydrolysis of the diester of binaphthol;193 and 3) forming inclusion complexes with suitable compounds.202122 The use of N-benzylcinchonidinium chloride to make inclusion complexes was reported by Tanaka and co-workers for obtaining one enantiomer of binaphthol.2324 Using acetonitrile as solvent, in which the inclusion complex has very low solubility, allows for the isolation of both enantiomers with high enantiomeric excess. This simple and efficient procedure represents a much better resolution for 1,1'-bi-2-naphthol.25

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